Stabilized capillary microjet and devices and methods for producing same

ABSTRACT

The invention is directed to a stable capillary microjet and a monodisperse aerosol formed when the microjet dissociates. A variety of devices and methods are disclosed which allow for the formation of a stream of a first fluid (e.g. a liquid) characterized by forming a stable capillary microjet over a portion of the stream wherein the microjet portion of the stream is formed by a second fluid (e.g. a gas). The second fluid is preferably in a different state from the first fluid—liquid-gas or gas-liquid combinations. However, the first and second fluids may be two different fluids in miscible in each other. The stable capillary microjet comprises a diameter d j  at a given point A in the stream characterized by the formula:          d   j     ≅         (       8                   ρ   1           π   2        Δ                   P   g         )       1   /   4            Q     1   /   2                         
     wherein d j  is the diameter of the stable microjet, ≈ indicates approximately equally to where an acceptable margin of error is ±10%, ρ 1  is the density of the liquid and ΔP g  is change in gas pressure of gas surrounding the stream at the point A.

CROSS-REFERENCES

This application is a continuation application of Ser. No. 09/853,153,May 11, 2001 which is a continuation of application Ser. No. 09/605,048,filed Jun. 27, 2000 now issued U.S. Pat. No. 6,234,402 which applicationis a continuation of application Ser. No. 09/192,091 filed Nov. 13, 1998now issued U.S. Pat. No. 6,116,516 which application is acontinuation-in-part of U.S. application Ser. No. 09/171,518 filed onApr. 21, 1999 now U.S. Pat. No. 6,119,953 which patent is incorporatedherein by reference and to which application is claimed priority under35 U.S.C. §120, and which is based on PCT/ES97/00034 filed Feb. 18, 1997and published as WO 97/43048 published Nov. 20, 1997 under 35 U.S.C.§365, said PCT application being the international version of SpanishApplication No. P9601101, May 13, 1996 to which priority is claimedunder 35 U.S.C. §§119 and 365. Still further, this application claimspriority to Spanish Application No. P9702654 filed Dec. 17, 1997 under35 U.S.C. §119.

FIELD OF THE INVENTION

This application generally relates to the field of finely directed fluidflow and more particularly to the creation of a stabilized capillarymicrojet which breaks up to form a monodisperse aerosol.

BACKGROUND OF THE INVENTION

Devices for creating finely directed streams of fluids and/or creatingaerosolized particles of a desired size are used in a wide range ofdifferent applications. For example, finely directed streams of ink forink jet printers, or directed streams of solutions containing biologicalmolecules for the preparation of microarrays. The production ofmonodisperse aerosols is also important for (1) aerosolized delivery ofdrugs to obtain deep even flow of the aerosolized particles into thelungs of patients; (2) aerosolizing fuel for delivery in internalcombustion engines to obtain rapid, even dispersion of any type of fuelin the combustion chamber; or (3) the formation of uniform sizedparticles which themselves have a wide range of uses including (a)making chocolate, which requires fine particles of a given size toobtain the desired texture or “mouth feel” in the resulting product, (b)making pharmaceutical products for timed release of drugs or to maskflavors and (c) making small inert particles which are used as standardsin tests or as a substrate onto which compounds to be tested, reacted orassayed are coated.

Although there is a need for creating finely directed streams of fluidsand for creating small spherical particles which are substantiallyuniform in size current methods suffer from a number of disadvantages.The invention described and disclosed herein is based on new discoveriesin the field of physics which make it possible to overcome disadvantagesof prior art devices and methods in an energy efficient manner.

SUMMARY OF THE INVENTION

The invention is directed to a stable capillary microjet and amonodisperse aerosol formed when the microjet dissociates. A variety ofdevices and methods are disclosed which allow for the formation of astream of a first fluid (e.g. a liquid) characterized by forming astable capillary microjet over a portion of the stream wherein themicrojet portion of the stream is formed by a second fluid (e.g. a gas).The second fluid is preferably in a different state from the firstfluid—liquid-gas or gas-liquid combinations. However, the first andsecond fluids may be two different fluids in miscible in each other. Thestable capillary microjet comprises a diameter d_(j) at a given point Ain the stream characterized by the formula:$d_{j} \cong {\left( \frac{8\quad \rho_{1}}{\pi^{2}\Delta \quad P_{g}} \right)^{1/4}Q^{1/2}}$

wherein d_(j) is the diameter of the stable microjet, ≅ indicatesapproximately equally to where an acceptable margin of error is ±10%, ρ₁is the density of the liquid and ΔP_(g) is change in gas pressure of gassurrounding the stream at the point A.

The microjet can have a diameter in the range of from about 1 micron toabout 1 mm and a length in the range of from 1 micron to 50 mm. Thestable jet is maintained, at least in part, by tangential viscousstresses exerted by the gas on the surface of the jet in an axialdirection of the jet. The jet is further characterized by a slightlyparabolic axial velocity profile and still further characterized by aWeber number (We) which is greater than 1 with the Weber number beingdefined by the formula: ${We} = \frac{\rho_{g}v_{g}^{2}d}{\gamma}$

wherein the ρ_(g) is the density of the gas, d is the diameter of thestable microjet, γ is the liquid-gas surface tension, and V_(g) ² is thevelocity of the gas squared.

Although the Weber number is greater than 1 when a stable microjet isobtained the Weber number should be less than 40 to obtain a desiredmonodisperse aerosol. Thus, desired results are obtained within theparameters of 1≦We≦40. Monodisperse aerosols of the invention have ahigh degree of uniformity in particle size. The particles arecharacterized by having the same diameter with a deviation in diameterfrom one particle to another in a range of about ±3% to about ±30%,preferably about ±3% to about ±10% and most preferably ±3% or less. Theparticles in an aerosol will have consistency in size but may beproduced to have a size in a range of about 0.1 micron to about 100microns.

An object of the invention is to provide a stream of a first fluid (e.g.a liquid) which stream is characterized by forming a stable capillarymicrojet over a portion of the stream wherein this stable capillarymicrojet portion of the stream is formed by a second fluid (e.g. a gas)moving at a velocity greater than that of the first fluid.

Another object of the invention is to provide a monodisperse aerosol ofliquid particles in air wherein the particles are characterized byhaving the same diameter with a deviation in diameter from one particleto another in a range of from about ±3% to about ±30% wherein theparticles are produced as a result of a break up of the stable capillarymicrojet.

An advantage of the invention is that the microjet of liquid flowsthrough an opening surrounded by a focusing funnel of gas so that liquiddoes not touch the peripheral area of the opening and therefor does notdeposit on the opening and cause clogging.

Another advantage of the invention is that the particles formed arehighly uniform in size and are created with a relatively small amount ofenergy.

A feature of the invention is that various parameters including theviscosities and velocities of the fluids can be chosen withconsideration to other adjusted parameters to obtain a supercriticalflow of liquid which results in the formation of the stable capillarymicrojet.

Another advantage of the invention is that the positions of the liquidand gas within the various embodiments of the invention can be changedin order to obtain a variety of different effects. For example, whenaqueous liquid forms a stable capillary microjet surrounded by afocusing funnel of gas which escapes into a surrounding gas at lowerpressure, aerosolized particles are formed. In another example a flowstream of gas focused by a surrounding focusing funnel of liquid whichflows outward into a liquid forms gas bubbles which are highly uniformand extremely small in size.

An advantage of the invention is that the embodiments can be used toform small gas bubbles which are uniform in size and sufficiently smallto provide for a high degree of diffusion of the gas present in thebubble into the surrounding liquid, thereby providing advantages such asoxygenating water or decontaminating gas.

These and other aspects, objects, features and advantages will becomeapparent to those skilled in the art upon reading this disclosure incombination with the figures provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view showing the basic components of oneembodiment of the invention with a cylindrical feeding needle as asource of formulation.

FIG. 1B is an enlarged schematic view of the cylindrical feeding needleshown in FIG. 1A.

FIG. 2 is a schematic view of another embodiment of the invention withtwo concentric tubes as a source of formulation.

FIG. 3 is a schematic view of yet another embodiment showing awedge-shaped planar source of formulation. FIG. 3a illustrates across-sectional side view of the planar feeding source and theinteraction of the fluids. FIG. 3b show a frontal view of the openingsin the pressure chamber, with the multiple openings through which theatomizate exits the device. FIG. 3c illustrates the channels that areoptionally formed within the planar feeding member. The channels arealigned with the openings in the pressure chamber.

FIG. 4 is a schematic view of a stable capillary microjet being formedand flowing through an exit opening to thereafter form a monodisperseaerosol.

FIG. 5 is a graph of data where 350 measured values of d_(j)/d_(n)versus Q/Q_(n) are plotted.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Before the present aerosol device and method are described, it is to beunderstood that this invention is not limited to the particularcomponents and steps described, as such may, of course, vary. It is alsoto be understood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting, since the scope of the present invention will be limited onlyby the appended claims.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “and,” and “the” include plural referents unless thecontext clearly dictates otherwise. Thus, for example, reference to “aparticle” includes a plurality of particles and reference to “a fluid”includes reference to a mixture of fluids, and equivalents thereof knownto those skilled in the art, and so forth.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present invention, the preferred methodsand materials are now described. All publications mentioned herein areincorporated herein by reference to disclose and describe the methodsand/or materials in connection with which the publications are cited.

The publications discussed herein are provided solely for theirdisclosure prior to the filing date of the present application. Nothingherein is to be construed as an admission that the present invention isnot entitled to antedate such publication by virtue of prior invention.Further, the dates of publication provided may be different from theactual publication dates which may need to be independently confirmed.

Device in General

Different embodiments are shown and described herein (see FIGS. 1, 2 and3) which could be used in producing the stable capillary microjet and/ora dispersion of particles which are substantially uniform in size.Although various embodiments are part of the invention, they are merelyprovided as exemplary devices which can be used to convey the essence ofthe invention, which is the formation of a stable capillary microjetand/or uniform dispersion of particles.

A basic device comprises (1) a means for supplying a first fluid and (2)a pressure chamber supplied with a second fluid which flows out of anexit opening in the pressure chamber. The exit opening of the pressurechamber is aligned with the flow path of the means for supplying thefirst fluid. The embodiments of FIGS. 1, 2 and 3 clearly show that therecan be a variety of different means for supplying the first fluid. Othermeans for supplying a first fluid flow stream will occur to thoseskilled in the art upon reading this disclosure.

Further, other configurations for forming the pressure chamber aroundthe means for supplying the fist fluid will occur to those skilled inthe art upon reading this disclosure. Such other embodiments areintended to be encompassed by the present invention provided the basicconceptual results disclosed here are obtained, i.e. a stable capillarymicrojet is formed and/or a dispersion of particle highly uniform insize is formed. To simplify the description of the invention, the meansfor supplying a first fluid is often referred to as a cylindrical tube(see FIG. 1) and the first fluid is generally referred to as a liquid.The liquid can be any liquid depending on the overall device which theinvention is used within. For example, the liquid could be a liquidformulation of a pharmaceutically active drug used to create an aerosolfor inhalation or, alternatively, it could be a hydrocarbon fuel used inconnection with a fuel injector for use on an internal combustion engineor heater or other device which burns hydrocarbon fuel. Further, forpurposes of simplicity, the second fluid is generally described hereinas being a gas and that gas is often preferably air. However, the firstfluid may be a gas and second fluid a liquid or both fluids may beliquid provided the first and second fluid are sufficiently differentfrom each other (immiscible) so as to allow for the formation of astable microjet of the first fluid moving from the supply means to anexit port of the pressure chamber. Notwithstanding these differentcombinations of gas-liquid, liquid-gas, and liquid-liquid, the inventionis generally described with a liquid formulation being expelled from thesupply means and forming a stable microjet due to interaction withsurrounding air flow focusing the microjet to flow out of an exit of thepressure chamber.

Formation of the microjet and its acceleration and ultimate particleformation are based on the abrupt pressure drop associated with thesteep acceleration experienced by the liquid on passing through an exitorifice of the pressure chamber which holds the second fluid (i.e. thegas). On leaving the chamber the flow undergoes a large pressuredifference between the liquid and the gas, which in turn produces ahighly curved zone on the liquid surface near the exit port of thepressure chamber and in the formation of a cuspidal point from which asteady microjet flows, provided the amount of the liquid withdrawnthrough the exit port of the pressure chamber is replenished. Thus, inthe same way that a glass lens or a lens of the eye focuses light to agiven point, the flow of the gas surrounds and focuses the liquid into astable microjet The focusing effect of the surrounding flow of gascreates a stream of liquid which is substantially smaller in diameterthan the diameter of the exit orifice of the pressure chamber. Thisallows liquid to flow out of the pressure chamber orifice withouttouching the orifice, providing advantages including (1) clogging of theexit orifice is virtually eliminated, (2) contamination of flow due tocontact with substances (e.g. bacteria or particulate residue) on theorifice opening is virtually eliminated, and (3) the diameter of thestream and the resulting particles are smaller than the diameter of theexit orifice of the chamber. This is particularly desirable because itis difficult to precisely engineer holes which are very small indiameter. Further, in the absence of the focusing effect (and formationa stable microjet) flow of liquid out of an opening will result inparticles which have about twice the diameter of the exit opening. Anadditional advantage is that the particles are not prone toagglomeration following exit from the chamber.

The description provided here generally indicates that the fluid leavesthe pressure chamber through an exit orifice surrounded by the gas andthereafter enters into a gaseous surrounding environment which may beair held at normal atmospheric pressure, or, alternatively, the gas(heated pressurized air) inside an internal combustion engine. However,when the first fluid is a gas and the second fluid is a liquid the fluidpresent outside of the chamber may also be a liquid. This configurationis particularly useful when it is necessary to create very small highlyuniform bubbles which are moved into a liquid surrounding exit openingof the pressure chamber. The need for the formation of very small highlyuniform bubbles into a gas occurs in a variety of different industrialapplications. For example, water needs to be oxygenated in a variety ofsituations including small at home fish tanks and large volumefisheries. The additional oxygen can aid the rate of growth of the fishand thereby improve production for the fishery. In another embodiment,oxygen or air bubbles can be forced into liquid sewage in order to aidin treatment. In yet another application of the invention, contaminatedgases such as a gas contaminated with a radioactive material can beformed into small uniformed bubbles and blown into a liquid where thecontamination in the gas will diffuse into the liquid, thereby cleaningthe gas. The liquid will, of course, occupy substantially less volumeand therefore be substantially easier to dispose of than contaminatedtoxic gas.

Those skilled in the art will recognize that variations on the differentembodiments disclosed below will be useful in obtaining particularlypreferred results. Specific embodiments of devices are now described.

Embodiment of FIGS. 1A and 1B

A first embodiment of the invention where the supply means is acylindrical feeding needle supplying liquid into a pressurized chamberof gas is described below with reference to FIGS. 1A and 1B.

The components of the embodiment of FIGS. 1A and 1B are as follows:

1. Feeding needle—also referred to generally as a fluid source and atube.

2. End of the feeding needle used to insert the liquid to be atomized.

3. Pressure chamber.

4. Orifice used as gas inlet.

5. End of the feeding needle used to evacuate the liquid to be atomized.

6. Orifice through which withdrawal takes place.

7. Atomizate (spray)—also referred to as aerosol.

D₀=diameter of the feeding needle; d₀ =diameter of the orifice throughwhich the microjet is passed; e=axial length of the orifice throughwhich withdrawal takes place; H=distance from the feeding needle to themicrojet outlet; P₀=pressure inside the chamber; P₁=atmosphericpressure.

Although the device can be configured in a variety of designs, thedifferent designs will all include the essential components shown inFIGS. 1A and 1B or components which perform an equivalent function andobtain the desired results. Specifically, a device of the invention willbe comprised of at least one source of a first fluid (e.g., a feedingneedle with an opening 2) into which a first fluid such as liquidflowable formulation can be fed and an exit opening 5 from which theformulation can be expelled. The feeding needle 1, or at least its exitopening 5, is encompassed by a pressure chamber 3. The chamber 3 hasinlet opening 4 which is used to feed a second fluid (e.g. a gas) intothe chamber 3 and an exit opening 6 through which gas from the pressurechamber and liquid formulation from the feeding needle 3 are expelled.When the first fluid is a liquid it is expelled into gas to create anaerosol. When the first fluid is a gas it is expelled into a liquid tocreate bubbles.

In FIGS. 1A and 1B, the feeding needle and pressure chamber areconfigured to obtain a desired result of producing an aerosol whereinthe particles are small and uniform in size or bubbles which are smalland uniform in size. The particles or bubbles have a size which is in arange of 0.1 to 100 microns. The particles of any given aerosol orbubbles will all have about the same diameter with a relative standarddeviation of 10% to 30% or more preferably 3% to 10%. Stating thatparticles of the aerosol have a particle diameter in a range of 1 to 5microns does not mean that different particles will have differentdiameters and that some will have a diameter of 1 micron while others of5 microns. The particles in a given aerosol will all (preferably about90% or more) have the same diameter 3% to 30%. For example, theparticles of a given aerosol will have a diameter of 2 microns 3% to10%. The same deviations are also correct for the formation of bubbles.

Such a monodisperse aerosol is created using the components andconfiguration as described above. However, other components andconfigurations will occur to those skilled in the art. The object ofeach design will be to supply fluid so that it creates a stablecapillary microjet which is accelerated and stabilized by tangential,viscous stress exerted by the second fluid on the first fluid surface.The stable microjet created by the second fluid leaves the pressurizedarea (e.g., leaves the pressure chamber and exits the pressure chamberorifice) and splits into particles or bubbles which have the desiredsize and uniformity.

The parameter window used (i.e. the set of special values for the liquidproperties, flow-rate used, feeding needle diameter, orifice diameter,pressure ratio, etc.) should be large enough to be compatible withvirtually any liquid (dynamic viscosities in the range from 10⁻¹ to 1 kgm⁻¹s⁻¹); in this way, the capillary microjet that emerges from the endof the feeding needle is absolutely stable and perturbations produced bybreakage of the jet cannot travel upstream. Downstream, the microjetsplits into evenly shaped drops simply by effect of capillaryinstability (see, for example, Rayleigh, “On the instability of jets”,Proc. London Math. Soc., 4-13, 1878), similar in a manner to a laminarcapillary jet falling from a half-open tap.

When the stationary, steady interface is created, the capillary jet thatemerges from the end of the drop at the outlet of the feeding point isconcentrically withdrawn into the nozzle. After the jet emerges from thedrop, the liquid is accelerated by tangential sweeping forces exerted bythe gas stream flowing on its surface, which gradually decreases the jetcross-section. Stated differently the gas flow acts as a lens andfocuses and stabilizes the microjet as it moves toward and into the exitorifice of the pressure chamber.

The forces exerted by the second fluid flow on the first fluid surfaceshould be steady enough to prevent irregular surface oscillations.Therefore, any turbulence in the gas motion should be avoided; even ifthe gas velocity is high, the characteristic size of the orifice shouldensure that the gas motion is laminar (similar to the boundary layersformed on the jet and on the inner surface of the nozzle or hole).

Stable Capillary Microjet

FIG. 4 illustrates the interaction of a liquid and a gas to formatomizate using the method of the invention. The feeding needle 60 has acircular exit opening 61 with an internal radius R₀ which feeds a liquid62 out of the end, forming a drop with a radius in the range of R₀, toR₀ plus the thickness of the wall of the needle. The exiting liquidforms an infinite amount of liquid streamlines 63 that interact with thesurrounding gas to form a stable cusp at the interface 64 of the twofluids. The surrounding gas also forms an infinite number of gasstreamlines 65, which interact with the exiting liquid to create avirtual focusing funnel 66. The exiting liquid is focused by thefocusing funnel 66 resulting in a stable capillary microjet 67, whichremains stable until it exits the opening 68 of the pressure chamber 69.After exiting the pressure chamber, the microjet begins to break-up,forming monodispersed particles 70.

The gas flow, which affects the liquid withdrawal and its subsequentacceleration after the jet is formed, should be very rapid but alsouniform in order to avoid perturbing the fragile capillary interface(the surface of the drop that emerges from the jet).

Liquid flows out of the end of a capillary tube and forms a small liquiddrop at the end. The tube has an internal radius R₀. The drop has aradius in a range of from R₀ to R₀ plus the structural thickness of thetube as the drop exits the tube, and thereafter the drop narrows incircumference to a much smaller circumference as is shown in theexpanded view of the tube (i.e. feeding needle) 5 as shown in FIGS. 1A,1B and 4.

As illustrated in FIG. 4, the exit opening 61 of the capillary tube 60is positioned close to an exit opening 68 in a planar surface of apressure chamber 69. The exit opening 68 has a minimum diameter D and isin a planar member with a thickness L. The diameter D is referred to asa minimum diameter because the opening may have a conical configurationwith the narrower end of the cone positioned closer to the source ofliquid flow. Thus, the exit opening may be a funnel-shaped nozzlealthough other opening configurations are also possible, e.g. an hourglass configuration. Gas in the pressure chamber continuously flows outof the exit opening. The flow of the gas causes the liquid drop expelledfrom the tube to decrease in circumference as the liquid moves away fromthe end of the tube in a direction toward the exit opening of thepressure chamber.

In actual use, it can be understood that the opening shape whichprovokes maximum gas acceleration (and consequently the most stable cuspand microjet with a given set of parameters) is a conically shapedopening in the pressure chamber. The conical opening is positioned withits narrower end toward the source of liquid flow.

The distance between the end 61 of the tube 60 and the beginning of theexit opening 68 is H. At this point it is noted that R₀, D, H and L areall preferably on the order of hundreds of microns. For example, R₀=400μm, D=150 μm, H=1 mm, L=300 μm. However, each could be {fraction(1/100)} to 100×these sizes.

The end of the liquid stream develops a cusp-like shape at a criticaldistance from the exit opening 68 in the pressure chamber 69 when theapplied pressure drop ΔP_(g) across the exit opening 68 overcomes theliquid-gas surface tension stresses γ/R* appearing at the point ofmaximum curvature—e.g. I/R* from the exit opening.

A steady state is then established if the liquid flow rate Q ejectedfrom the drop cusp is steadily supplied from the capillary tube. This isthe stable capillary cusp which is an essential characteristic of theinvention needed to form the stable microjet. More particularly, asteady, thin liquid jet with a typical diameter d_(j) is smoothlyemitted from the stable cusp-like drop shape and this thin liquid jetextends over a distance in the range of microns to millimeters. Thelength of the stable microjet will vary from very short (e.g. 1 micron)to very long (e.g. 50 mm) with the length depending on the (1) flow-rateof the liquid and (2) the Reynolds number of the gas stream flowing outof the exit opening of the pressure chamber. The liquid jet is thestable capillary microjet obtained when supercritical flow is reached.This jet demonstrates a robust behavior provided that the pressure dropΔP_(g) applied to the gas is sufficiently large compared to the maximumsurface tension stress (on the order of γ/d_(j)) that act at theliquid-gas interface. The jet has a slightly parabolic axial velocityprofile which is, in large part, responsible for the stability of themicrojet. The stable microjet is formed without the need for otherforces, i.e. without adding force such as electrical forces on a chargedfluid. However, for some applications it is preferable to add charge toparticles, e.g. to cause the particles to adhere to a given surface. Theshaping of liquid exiting the capillary tube by the gas flow forming afocusing funnel creates a cusp-like meniscus resulting in the stablemicrojet. This is a fundamental characteristic of the invention.

The fluid stream flowing from the tube has substantially more densityand develops substantially more inertia as compared to the gas, whichhas lower viscosity than the liquid. These characteristics contribute tothe formation of the stable capillary jet. The stable capillary microjetis maintained stably for a significant distance in the direction of flowaway from the exit from the tube. The liquid is, at this point,undergoing “supercritical flow.” The microjet eventually destabilizesdue to the effect of surface tension forces. Destabilization resultsfrom small natural perturbations moving downstream, with the fastestgrowing perturbations being those which govern the break up of themicrojet, eventually creating a monodisperse (a uniform sized) aerosol70 as shown in FIG. 4.

The microjet, even as it initially destabilizes, passes out of the exitorifice of the pressure chamber without touching the peripheral surfaceof the exit opening. This provides an important advantage of theinvention which is that the exit opening 68 (which could be referred toas a nozzle) will not clog from residue and/or deposits of the liquid.Clogging is a major problem with very small nozzles and is generallydealt with by cleaning or replacing the nozzle. When fluid contacts thesurfaces of a nozzle opening some fluid will remain in contact with thenozzle when the flow of fluid is shut off. The liquid remaining on thenozzle surface evaporates leaving a residue. After many uses over timethe residue builds up and clogging takes place. The present inventionsubstantially reduces or eliminates this clogging problem.

Mathematics of a Stable Microjet

Cylindrical coordinates (r,z) are chosen for making a mathematicalanalysis of a stable microjet, i.e. liquid undergoing “supercriticalflow.” The cusp-like meniscus formed by the liquid coming out of thetube is pulled toward the exit of the pressure chamber by a pressuregradient created by the flow of gas.

The cusp-like meniscus formed at the tube's mouth is pulled towards thehole by the pressure gradient created by the gas stream. From the cuspof this meniscus, a steady liquid thread with the shape of radius r=ξ iswithdrawn through the hole by the action of both the suction effect dueto ΔP_(g), and the tangential viscous stresses τ_(s) exerted by the gason the jet's surface in the axial direction. The averaged momentumequation for this configuration may be written $\begin{matrix}{{{\frac{d}{d_{z}}\left\lbrack {P_{1} + \frac{\rho_{1}\quad Q^{2}}{2\quad \pi^{2}\xi^{4}}} \right\rbrack} = \frac{2\tau_{s}}{\xi}},} & (1)\end{matrix}$

where Q is the liquid flow rate upon exiting the feeding tube, P₁ is theliquid pressure, and ρ₁ is the liquid density, assuming that the viscousextensional term is negligible compared to the kinetic energy term, aswill be subsequently justified. In addition, liquid evaporation effectsare neglected. The liquid pressure P₁ is given by the capillaryequation.

P ₁ =P _(q)+γ/ξ.  (2)

where γ is the liquid-gas surface tension. As shown in the Examples, thepressure drop ΔP_(g) is sufficiently large as compared to the surfacetension stress γ/ξ to justify neglecting the latter in the analysis.This scenario holds for the whole range of flow rates in which themicrojet is absolutely stable. In fact, it will be shown that, for agiven pressure drop ΔP_(g), the minimum liquid flow rate that can besprayed in steady jet conditions is achieved when the surface tensionstress γ/ξ is of the order of the kinetic energy of the liquidρ₁Q²/(2π²ξ⁴), since the surface tension acts like a “resistance” to themotion (it appears as a negative term in the right-hand side term of Eq.(1)) Thus, $\begin{matrix}{\left. Q_{\min} \right.\sim\left( \frac{{\gamma d}_{j}^{3}}{\rho_{1}} \right)^{1/2}} & (3)\end{matrix}$

For sufficiently large flow rates Q compared to Q_(min), the simplifiedaveraged momentum equation in the axial direction can be expressed as$\begin{matrix}{{{\frac{d}{d_{z}}\left( \frac{\rho_{1}\quad Q^{2}}{2\quad \pi^{2}\xi^{4}} \right)} = {\frac{{dP}_{g}}{d_{z}} + \frac{2\tau_{s}}{\xi}}},} & (4)\end{matrix}$

where one can identify the two driving forces for the liquid flow on theright-hand side. This equation can be integrated provided the followingsimplification is made: if one uses a thin plate with thickness L of theorder or smaller than the hole's diameter D (which minimizes downstreamperturbations in the gas flow), the pressure gradient up to the holeexit is on the average much larger than the viscous shear term 2τ_(g)/ξ, owning to the surface stress. On the other hand, the axial viscousterm is of the order O[μ²Q /D²d_(j) ²], since the hole diameter D isactually the characteristic distance associated with the gas flow at thehole's entrance in both the radial and axial directions. This term isvery small compared to the pressure gradient in real situations,provided that ΔP_(g)>>μ²/D²p₁ (which holds, e.g., for liquids withviscosities as large as 100 cpoises, using hole diameters and pressuredrops as small as D ˜10 μm and ΔP_(g)≧100 mbar). The neglect of allviscous terms in Eq. (4) is then justified. Notice that in this limit onthe liquid flow is quasi-isentropic in the average (the liquid almostfollows Bernoulli equation) as opposed to most micrometric extensionalflows. Thus, integrating (4) from the stagnation regions-of both fluidsup to the exit, one obtains a simple and universal expression for thejet diameter at the hole exit: $\begin{matrix}{{d_{j} \simeq {\left( \frac{8\rho_{1}}{\pi^{2}\Delta \quad P_{g}} \right)^{1/4}Q^{1/2}}},} & (5)\end{matrix}$

which for a given pressure drop ΔP_(g) is independent of geometricalparameters (hole and tube diameters, tube-hole distance, etc.), liquidand gas viscosities, and liquid-gas surface tension. This diameterremains almost constant up to the breakup point since the gas pressureafter the exit remains constant.

Monodisperse Particles

Above the stable microjet undergoing “supercritical flow” is describedand it can be seen how this aspect of the invention can be made use ofin a variety of industrial applications—particularly where the flow ofliquid through small holes creates a clogging problem. An equallyimportant aspect of the invention is obtained after the microjet leavesthe pressure chamber.

When the microjet exits the pressure chamber the liquid pressure P₁becomes (like the gas pressure P_(g)) almost constant in the axialdirection, and the jet diameter remains almost constant up to the pointwhere it breaks up by capillary instability. Defining a Weber number We=(ρ_(g)ν_(g) ²d_(j))/γ=2ΔP_(g)d_(j)/γ (where ν_(g) is the gas velocitymeasured at the orifice), below a certain experimental value We_(c)˜40the breakup mode is axisymmetric and the resulting droplet stream ischaracterized by its monodipersity provided that the fluctuations of thegas flow do not contribute to droplet coalescence (these fluctuationsoccur when the as stream reaches a fully developed turbulent profilearound the liquid jet breakup region). Above this We_(c) value, sinuousnonaxisymmetric disturbances, coupled to the axisymmetric ones, becomeapparent. For larger We numbers, the nonlinear growth rate of thesinuous disturbances seems to overcome that of the axisymmetricdisturbances. The resulting spray shows significant polydispersity inthis case. Thus, it can be seen that by controlling parameters to keepthe resulting Weber number to 40 or less, allows the particles formed tobe all substantially the same size. The size variation is about ±3% to±30% and move preferably ±3% to ±10%. These particles can have a desiredsize e.g. 0.1 microns to 50 microns.

The shed vorticity influences the breakup of the jet and thus theformation of the particles. Upstream from the hole exit, in theaccelerating region, the gas stream is laminar. Typical values of theReynolds number range from 500 to 6000 if a velocity of the order of thespeed of sound is taken as characteristic of the velocity of the gas.Downstream from the hole exit, the cylindrical mixing layer between thegas stream and the stagnant gas becomes unstable by the classicalKelvin-Helmholtz instability. The growth rate of the thickness of thislayer depends on the Reynolds number of the flow and ring vortices areformed at a frequency of the order of ν_(g)/D, where D is the holediameter. Typical values of ν_(g) and D as those found in ourexperimental technique lead to frequencies or the order of MHZ which arecomparable to the frequency of drop production (of order of t_(b) ⁻¹).

Given the liquid flow rate and the hole diameter, a resonance frequencywhich depends on the gas velocity (or pressure difference driving thegas stream) can be adjusted (tuned) in such a way that vortices act as aforcing system to excite perturbations of a determined wavelength on thejet surface. Experimental results obtained clearly illustrates thedifferent degree of coupling between the two gas-liquid coaxial jets. Inone set of experimental results the particle sizes are shown to have aparticle size of about 5.7 microns with a standard deviation of 12%.This results when the velocity of the gas has been properly tuned tominimize the dispersion in the size of droplets resulting from the jetbreakup. In this case, the flow rate of the liquid jet and its diameterare 0.08 μl s⁻¹ and 3 μm, respectively. Data have been collected using aMASTERSIZER from MALVERN Instruments. As the degree of couplingdecreases, perturbations at the jet surface of different wavelengthsbecome excited and, as it can be observed rom the size distributions,the dispersion of the spray increases.

It is highly desirable in a number of different industrial applicationsto have particles which are uniform in size or to create aerosols ofliquid particles which are uniform in size. For example, particles of aliquid formation containing a pharmaceutically active drug could becreated and designed to have a diameter of about 2 microns ±3%. Theseparticles could be inhaled into the lungs of a patient forintrapulmonary drug delivery. Moreover, particle size can be adjusted totarget a particular area of the respiratory tract

The gas flow should be laminar in order to avoid a turbulent regime -turbulent fluctuations in the gas flow which have a high frequency andwould perturb the liquid-gas interface. The Reynolds numbers reached atthe orifice are ${Re} = {\frac{v_{g}d_{0}}{v_{g}} \sim 4000}$

where ν_(g) is the kinematic viscosity of the gas. Even though thisnumber is quite high, there are large pressure gradients downstream (ahighly convergent geometry), so that a turbulent regime is very unlikelyto develop.

The essential difference from existing pneumatic atomizers (whichpossess large Weber numbers) and the present invention is that the aimof the present invention is not to rupture the liquid-gas interface butthe opposite, i.e. to increase the stability of the interface until acapillary jet is obtained. The jet, which will be very thin provided thepressure drop resulting from withdrawal is high enough, splits intodrops the sizes of which are much more uniform than those resulting fromdisorderly breakage of the liquid-gas interface in existing pneumaticatomizers.

The proposed atomization system obviously requires delivery of theliquid to be atomized and the gas to be used in the resulting spray.Both should be fed at a rate ensuring that the system lies within thestable parameter window. Multiplexing is effective when the flow-ratesneeded exceed those on an individual cell. More specifically, aplurality of feeding sources or feeding needles may be used to increasethe rate at which aerosols are created. The flow-rates used should alsoensure the mass ratio between the flows is compatible with thespecifications of each application.

The gas and liquid can be dispensed by any type of continuous deliverysystem (e.g. a compressor or a pressurized tank the former and avolumetric pump or a pressurized bottle the latter). If multiplexing isneeded, the liquid flow-rate should be as uniform as possible amongcells; this may entail propulsion through several capillary needles,porous media or any other medium capable of distributing a uniform flowamong different feeding points.

Each individual atomization device should consist of a feeding point (acapillary needle, a point with an open microchannel, a microprotuberanceon a continuous edge, etc.) 0.002-2 mm (but, preferentially 0.01-0.4 mm)in diameter, where the drop emerging from the microjet can be anchored,and a small orifice 0.002-2 mm (preferentially 0.01-0.25 mm) in diameterfacing the drop and separated 0.01-2 mm (preferentially 0.2-0.5 mm) fromthe feeding point. The orifice communicates the withdrawal gas aroundthe drop, at an increased pressure, with the zone where the atomizate isproduced, at a decreased pressure. The atomizer can be made from avariety of materials (metal, polymers, ceramics, glass).

FIGS. 1A and 1B depict a tested prototype where the liquid to beatomized is inserted through one end of the system 2 and the propellinggas in introduced via the special inlet 4 in the pressure chamber 3. Theprototype was tested at gas feeding rates from 100 to 2000 mBar abovethe atmospheric pressure P_(a) at which the atomized liquid wasdischarged. The whole enclosure around the feeding needle 1 was at apressure P₀>P_(a). The liquid feeding pressure, Pi, should always beslightly higher than the gas propelling pressure, P₀. Depending on thepressure drop in the needle and the liquid feeding system, the pressuredifference (P₁−P₀>0) and the flow-rate of the liquid to be atomized, Q,are linearly related provided the flow is laminar—which is indeed thecase with this prototype. The critical dimensions are the distance fromthe needle to the plate (H), the needle diameter (D₀), the diameter ofthe orifice through which the microjet 6 is discharged (d₀) and theaxial length, e, of the orifice (i.e. the thickness of the plate wherethe orifice is made). In this prototype, H was varied from 0.3 to 0.7 mmon constancy of the distances (D₀=0.45 mm, d₀−0.2 mm) and e−0.5 mm. Thequality of the resulting spray 7 did not vary appreciably with changesin H provided the operating regime (ie. stationary drop and microjet)was maintained. However, the system stability suffered at the longer Hdistances (about 0.7 mm). The other atomizer dimensions had no effect onthe spray or the prototype functioning provided the zone around theneedle (its diameter) was large enough relative to the feeding needle.As explained further below it is possible to obtain a stable capillarymicrojet which does not disassociate into a monodisperse aerosol.However, by adjusting parameters which relate to the Weber number astable microjet is formed which disassociates to monodisperse aerosol.

Weber Number

Adjusting parameters to obtain a stable capillary microjet and controlits breakup into monodisperse particle is governed by the Weber numberand the liquid-to-gas velocity ratio or α which equal V₁/V_(g). TheWeber number or “We” is defined by the following equation:${We} = \frac{\rho_{g}v_{g}^{2}d}{\gamma}$

wherein ρ_(g) is the density of the gas, d is the diameter of the stablemicrojet, γ is the liquid-gas surface tension, and V_(g) ² is thevelocity of the gas squared.

When carrying out the invention the parameters should be adjusted sothat the Weber number is greater than 1 in order to produce a stablecapillary microjet. However, to obtain a particle dispersion which ismonodisperse (i.e. each particle has the same size ±3 to ±30%) theparameters should be adjusted so that the Weber number is less than 40.The monodisperse aerosol is obtained with a Weber number in a range ofabout 1 to about 40 when the breaking time is sufficiently small toavoid non-symmetric perturbations. (1≦We≦40)

Ohnesorge Number

A measure of the relative importance of viscosity on the jet breakup canbe estimated from the Ohnesorge number defined as the ratio between twocharacteristic times: the viscous time t_(v) and the breaking timet_(b). The breaking time t_(b) is given by [see Rayleigh (1878)]$\begin{matrix}{t_{b} \sim {\left( \frac{\rho_{1}d^{2}}{\gamma} \right)^{1/2}.}} & (2)\end{matrix}$

Perturbations on the jet surface are propagated inside by viscousdiffusion in times t_(v) of the order of

where μ₁ is the viscosity of the liquid. Then, the Ohnesorge number, Oh,results $\begin{matrix}{{Oh} = {\frac{\mu_{1}}{\left( {\rho_{1}\gamma \quad d} \right)^{1/2}}.}} & (4)\end{matrix}$

If this ratio is much smaller than unity viscosity plays no essentialrole in the phenomenon under consideration. Since the maximum value ofthe Ohnesorge number in actual experiments conducted is as low as3.7×10⁻², viscosity plays no essential role during the process of jetbreakup.

Embodiment of FIG. 2

A variety of configurations of components and types of fluids willbecome apparent to those skilled in the art upon reading thisdisclosure. These configurations and fluids are encompassed by thepresent invention provided they can produce a stable capillary microjetof a first fluid from a source to an exit port of a pressure chambercontaining a second fluid. The stable microjet is formed by the firstfluid flowing from the feeding source to the exit port of the pressurechamber being accelerated and stabilized by tangential viscous stressexerted by the second fluid in the pressure chamber on the surface ofthe first fluid forming the microjet. The second fluid forms a focusingfunnel when a variety of parameters are correctly tuned or adjusted. Forexample, the speed, pressure, viscosity and miscibility of the first andsecond fluids are chosen to obtain the desired results of a stablemicrojet of the first fluid focused into the center of a funnel formedwith the second fluid. These results are also obtained by adjusting ortuning physical parameters of the device, including the size of theopening from which the first fluid flows, the size of the opening fromwhich both fluids exit, and the distance between these two openings.

The embodiment of FIGS. 1A and 1B can, itself, be arranged in a varietyof configurations. Further, as indicated above, the embodiment mayinclude a plurality of feeding needles. A plurality of feeding needlesmay be configured concentrically in a single construct, as shown in FIG.2.

The components of the embodiment of FIG. 2 are as follows:

21. Feeding needle—tube or source of fluid.

22. End of the feeding needle used to insert the liquids to be atomized.

23. Pressure chamber.

24. Orifice used as gas inlet.

25. End of the feeding needle used to evacuate the liquid to beatomized.

26. Orifice through which withdrawal takes place.

27. Atomizate (spray) or aerosol.

28. First liquid to be atomized (inner core of particle).

29. Second liquid to be atomized (outer coating of particle).

30. Gas for creation of microjet.

31. Internal tube of feeding needle.

32. External tube of feeding needle.

D=diameter of the feeding needle; d=diameter of the orifice throughwhich the microjet is passed; e=axial length of the orifice throughwhich withdrawal takes place; H=distance from the feeding needle to themicrojet outlet; γ=surface tension; P₀=pressure inside the chamber;P_(a)=atmospheric pressure.

The embodiment of FIG. 2 is preferably used when attempting to form aspherical particle of one substance coated by another substance. Thedevice of FIG. 2 is comprised of the same basic component as per thedevice of FIGS. 1A and 1B and further includes a second feeding source32 which is positioned concentrically around the first cylindricalfeeding source 31. The second feeding source may be surrounded by one ormore additional feeding sources with each concentrically positionedaround the preceding source. The outer coating may be used for a varietyof purposes, including: coating particles to prevent small particlesfrom sticking together; to obtain a sustained release effect of theactive compound (e.g. a pharmaceutically active drug) inside, and/or tomask flavors; and to protect the stability of another compound (e.g. apharmaceutically active drug) contained therein.

The process is based on the microsuction which the liquid-gas orliquid-liquid interphase undergoes (if both are immiscible), when saidinterphase approaches a point beginning from which one of the fluids issuctioned off while the combined suction of the two fluids is produced.The interaction causes the fluid physically surrounded by the other toform a capillary microjet which finally breaks into spherical drops. Ifinstead of two fluids (gas-liquid), three or more are used that flow ina concentric manner by injection using concentric tubes, a capillary jetcomposed of two or more layers of different fluids is formed which, whenit breaks, gives rise to the formation of spheres composed of severalapproximately concentric spherical layers of different fluids. The sizeof the outer sphere (its thickness) and the size of the inner sphere(its volume) can be precisely adjusted. This can allow the manufactureof coated particles for a variety of end uses. For example the thicknessof the coating can be varied in different manufacturing events to obtaincoated particles which have gradually decreasing thicknesses to obtain acontrolled release effect of the contents, e.g. a pharmaceuticallyactive drug. The coating could merely prevent the particles fromdegrading, reacting, or sticking together.

The method is based on the breaking of a capillary microjet composed ofa nucleus of one liquid or gas and surrounded by another or otherliquids and gases which are in a concentric manner injected by a specialinjection head, in such a way that they form a stable capillary microjetand that they do not mix by diffusion during the time between when themicrojet is formed and when it is broken. When the capillary microjet isbroken into spherical drops under the proper operating conditions, whichwill be described in detail below, these drops exhibit a sphericalnucleus, the size and eccentricity of which can be controlled.

In the case of spheres containing two materials, the injection head 25consists of two concentric tubes with an external diameter on the orderof one millimeter. Through the internal tube 31 is injected the materialthat will constitute the nucleus of the microsphere, while between theinternal tube 31 and the external tube 32 the coating is injected. Thefluid of the external tube 32 joins with the fluid of tube 31 as thefluids exit the feeding needle, and the fluids (normally liquids) thusinjected are accelerated by a stream of gas that passes through a smallorifice 24 facing the end of the injection tubes. When the drop inpressure across the orifice 24 is sufficient, the liquids form acompletely stationary capillary microjet, if the quantities of liquidsthat are injected are stationary. This microjet does not touch the wallsof the orifice, but passes through it wrapped in the stream of gas orfunnel formed by gas from the tube 32. Because the funnel of gas focusesthe liquid, the size of the exit orifice 26 does not dictate the size ofthe particles formed.

When the parameters are correctly adjusted, the movement of the liquidis uniform at the exit of the orifice 26 and the viscosity forces aresufficiently small so as not to alter either the flow or the propertiesof the liquids; for example, if there are biochemical molecularspecimens having a certain complexity and fragility, the viscous forcesthat would appear in association with the flow through a micro-orificemight degrade these substances.

FIG. 2 shows a simplified diagram of the feeding needle 21, which iscomprised of the concentric tubes 30, 31 through the internal andexternal flows of the fluids 28, 29 that are going to compose themicrospheres comprised of two immiscible fluids. The difference inpressures P₀−P₁(P₀−P₁) through the orifice 26 establishes a flow of gaspresent in the chamber 23 and which is going to surround the microjet atits exit. The same pressure gradient that moves the gas is the one thatmoves the microjet in an axial direction through the hole 26, providedthat the difference in pressures P₀−P₁ is sufficiently great incomparison with the forces of surface tension, which create an adversegradient in the direction of the movement.

There are two limitations for the minimum sizes of the inside andoutside jets that are dependent (a) on the surface tensions γ1 of theoutside liquid 29 with the gas 30 and γ2 of the outside liquid 29 withthe inside liquid 28, and (b) on the difference in pressures ΔP=P₀−P₁through the orifice 26. In the first place, the jump in pressures APmust be sufficiently great so that the adverse effects of the surfacetension are minimized. This, however, is attained for very modestpressure increases: for example, for a 10 micron jet of a liquid havinga surface tension of 0.05 N/m (tap water), the necessary minimum jump inpressure is in the order of 0.05 (N/m)/0.00001 m=ΔP=50 mBar. But, inaddition, the breakage of the microjet must be regular and axisymmetric,so that the drops will have a uniform size, while the extra pressure ΔPcannot be greater than a certain value that is dependent on the surfacetension of the outside liquid with the gas γ1 and on the outsidediameter of the microjet. It has been experimentally shown that thisdifference in pressures cannot be greater than 20 times the surfacetension γ1 divided by the outside radius of the microjet.

Therefore, given some inside and outside diameters of the microjet,there is a range of operating pressures between a minimum and a maximum;nonetheless, experimentally the best results are obtained for pressuresin the order of two to three times the minimum.

The viscosity values of the liquids must be such that the liquid withthe greater viscosity μ_(max) verifies, for a diameter d of the jetpredicted for this liquid and a difference through the orifice ΔP, theinequality: $\mu_{\max} \leq \frac{\Delta \quad {Pd}^{2}D}{Q}$

With this, the pressure gradients can overcome the extensional forces ofviscous resistance exerted by the liquid when it is suctioned toward theorifice.

Moreover, the liquids must have very similar densities in order toachieve the concentricity of the nucleus of the microsphere, since therelation of velocities between the liquids moves according to the squareroot of the densities v1/v2=(ρ2/ρ1)^(½) and both jets, the inside jetand the outsidejet, must assume the most symmetrical configurationpossible, which does not occur if the liquids have different velocities(FIG. 2). Nonetheless, it has been experimentally demonstrated that, onaccount of the surface tension γ2 between the two liquids, the nucleustends to migrate toward the center of the microsphere, within prescribedparameters.

When two liquids and gas are used on the outside, the distance betweenthe planes of the mouths of the concentric tubes can vary, without thecharacteristics of the jet being substantially altered, provided thatthe internal tube 31 is not introduced into the external one 32 morethan one diameter of the external tube 32 and provided that the internaltube 31 does not project more than two diameters from the external tube32. The best results are obtained when the internal tube 31 projectsfrom the external one 32 a distance substantially the same as thediameter of the internal tube 31. This same criterion is valid if morethan two tubes are used, with the tube that is surrounded (inner tube)projecting beyond the tube that surrounds (outer tube) by a distancesubstantially the same as the diameter of the first tube

The distance between the plane of the internal tube 31 (the one thatwill normally project more) and the plane of the orifice may varybetween zero and three outside diameters of the external tube 32,depending on the surface tensions between the liquids and with the gas,and on their viscosity values. Typically, the optimal distance is foundexperimentally for each particular configuration and each set of liquidsused.

The proposed atomizing system obviously requires fluids that are goingto be used in the resulting spray to have certain flow parameters.Accordingly, flows for this use must be:

Flows that are suitable so that the system falls within the parametricwindow of stability. Multiplexing (i.e. several sets of concentrictubes) may be used, if the flows required are greater than those of anindividual cell.

Flows that are suitable so that the mass relation of the fluids fallswithin the specifications of each application. Of course, a greater flowof gas may be supplied externally by any means in specific applications,since this does not interfere with the functioning of the atomizer.

If the flows are varied, the characteristic time of this variation mustbe less than the hydrodynamic residence times of liquid and gas in themicrojet, and less than the inverse of the first natural oscillationfrequency of the drop formed at the end of the injection needle.

Therefore, any means for continuous supply of gas (compressors, pressuredeposits, etc.) and of liquid (volumetric pumps, pressure bottles) maybe used. If multiplexing is desired, the flow of liquid must be ashomogeneous as possible between the various cells, which may requireimpulse through multiple capillary needles, porous media, or any othermedium capable of distributing a homogeneous flow among differentfeeding points.

Each atomizing device will consist of concentric tubes 31, 32 with adiameter ranging between 0.05 and 2 mm, preferably between 0.1 and 0.4mm, on which the drop from which the microjet emanates can be anchored,and a small orfice (between 0.001 and 2 mm in diameter, preferablybetween 0.1 and 0.25 mm), facing the drop and separated from the pointof feeding by a distance between 0.001 and 2 mm, preferably between 0.2and 0.5 mm. The orifice puts the suction gas that surrounds the drop, athigher pressure, in touch with the area in which the atomizing is to beattained, at lower pressure.

Embodiment of FIG. 3

The embodiments of FIGS. 1 and 2 are similar in a number of ways. Bothhave a feeding piece which is preferably in the form of a feeding needlewith a circular exit opening. Further, both have an exit port in thepressure chamber which is positioned directly in front of the flow pathof fluid out of the feeding source. Precisely maintaining the alignmentof the flow path of the feeding source with the exit port of thepressure chamber can present an engineering challenge particularly whenthe device includes a number of feeding needles. The embodiment of FIG.3 is designed to simplify the manner in which components are aligned.The embodiment of FIG. 3 uses a planar feeding piece (which by virtue ofthe withdrawal effect produced by the pressure difference across a smallopening through which fluid is passed) to obtain multiple microjetswhich are expelled through multiple exit ports of a pressure chamberthereby obtaining multiple aerosol streams. Although a single planarfeeding member as shown in FIG. 3 it, of course, is possible to producea device with a plurality of planar feeding members where each planarfeeding member feeds fluid to a linear array of outlet orifices in thesurrounding pressure chamber. In addition, the feeding member need notbe strictly planar, and may be a curved feeding device comprised of twosurfaces that maintain approximately the same spatial distance betweenthe two pieces of the feeding source. Such curved devices may have anylevel of curvature, e.g. circular, semicircular, elliptical,hemielliptical, etc.

The components of the embodiment of FIG. 3 are as follows:

41. Feeding piece.

42. End of the feeding piece used to insert the fluid to be atomized.

43. Pressure chamber.

44. Orifice used as gas inlet

45. End of the feeding needle used to evacuate the liquid to beatomized.

46. Orifices through which withdrawal takes place.

47. Atomizate (spray) or aerosol.

48. first fluid containing material to be atomized.

49. second fluid for creation of microjet.

50. wall of the propulsion chamber facing the edge of the feeding piece.

51. channels for guidance of fluid through feeding piece.

d_(j)=diameter of the microjet formed; ρ_(A)=liquid density of firstfluid (48); ρ_(B)=liquid density of second fluid (49); ν_(A)=velocity ofthe first liquid (48); ν_(B)=velocity of the second liquid (49); e=axiallength of the orifice through which withdrawal takes place; H=distancefrom the feeding needle to the microjet outlet; P₀=pressure inside thechamber;

Δp_(g)=change in pressure of the gas; P_(a)=atmospheric pressure;Q=volumetric flow rate

The proposed dispersing device consists of a feeding piece 41 whichcreates a planar feeding channel through which a where a first fluid 48flows. The flow is preferably directed through one or more channels ofuniform bores that are constructed on the planar surface of the feedingpiece 41. A pressure chamber 43 that holds the propelling flow of asecond liquid 49, houses the feeding piece 41 and is under a pressureabove maintained outside the chamber wall 50. One or more orifices,openings or slots (outlets) 46 made in the wall 52 of the propulsionchamber face the edge of the feeding piece. Preferably, each bore orchannel of the feeding piece 41 has its flow path substantially alignedwith an outlet 46.

Formation of the microjet and its acceleration are based on the abruptpressure drop resulting from the steep acceleration undergone by thesecond fluid 49 on passing through the orifice 46, similarly to theprocedure described above for embodiments of FIGS. 1 and 2 when thesecond fluid 49 is a gas.

When the second fluid 49 is a gas and the first fluid 48 is a liquid,the microthread formed is quite long and the liquid velocity is muchsmaller than the gas velocity. In fact, the low viscosity of the gasallows the liquid to flow at a much lower velocity; as a result, themicrojet is actually produced and accelerated by stress forces normal tothe liquid surface, i.e. pressure forces. Hence, one effectiveapproximation to the phenomenon is to assume that the pressuredifference established will result in the same kinetic energy per unitvolume for both fluids (liquid and gas), provided gas compressibilityeffects are neglected. The diameter d_(j) of the microjet formed from aliquid density ρ₁ that passes at a volumetric flow-rate Q through anorifice across which a pressure difference ΔP_(g) exists will be givenby$d_{j} \cong {\left( \frac{8\quad \rho_{1}}{\pi^{2}\Delta \quad P_{g}} \right)^{1/4}Q^{1/2}}$

See Gañàn-Calvo, Physical Review Letters, 80:285-288 (1998).

The relation between the diameter of the microjet, d_(j), and that ofthe resulting drops, {overscore (d)}, depends on the ratio betweenviscous forces and surface tension forces on the liquid on the one hand,and between dynamic forces and surface tension forces on the gas on theother (i.e. on the Ohnesorge and Weber numbers, respectively) (Hinds(Aerosol Technology, John & Sons, 1982), Lefevre (Atomization andSprays, Hemisphere Pub. Corp., 1989) and Bayvel & Orzechowski (LiquidAtonzization, Taylor & Francis, 1993)). At moderate to low gasvelocities and low viscosities the relation is roughly identical withthat for capillarity instability developed by Rayleigh:

{overscore (d)}=1.89d_(j)

Because the liquid microjet is very long, at high liquid flow-rates thetheoretical rupture point lies in the turbulent zone created by the gasjet, so turbulent fluctuations in the gas destabilize or rupture theliquid microjet in a more or less uneven manner. As a result, thebenefits of drop size uniformity are lost.

On the other hand, when the second fluid 49 is a liquid and the firstfluid 48 is a gas, the facts that the liquid is much more viscous andthat the gas is much less dense virtually equalize the fluid and gasvelocities. The gas microthread formed is much shorter; however, becauseits rupture zone is almost invariably located in a laminar flowingstream, dispersion in the size of the microbubbles formed is almostalways small. At a volumetric gas flow-rate Q_(g) and a liquidoverpressure ΔP₁, the diameter of the gas microjet is given by$d_{j} \cong {\left( \frac{8\rho_{1}}{\pi^{2}\Delta \quad P_{1}} \right)^{1/4}Q_{g}^{1/2}}$

The low liquid velocity and the absence of relative velocities betweenthe liquid and gas lead to the Rayleigh relation between the diametersof the microthread and those of the bubbles (i.e. d=1.89d_(j)).

If both fluids 48, 49 are liquid and scarcely viscous, then theirrelative velocities will be given by$\frac{v_{A}}{v_{B}} = \left( \frac{\rho_{B}}{\rho_{A}} \right)^{1/2}$

The diameter of a microjet of the first liquid at a volumetric flow-rateof A Q_(A) and an overpressure of BΔP_(B) will be given by$d_{j} \cong {\left( \frac{8\rho_{A}}{\pi^{2}\Delta \quad P_{B}} \right)^{1/4}Q_{A}^{1/2}}$

At viscosities such that the velocities of both fluids 48, 49 willrapidly equilibrate in the microjet, the diameter of the microjet of thefirst liquid will be given by$d_{j} \cong {\left( \frac{8\rho_{B}}{\pi^{2}\Delta \quad P_{B}} \right)^{1/4}Q_{A}^{1/2}}$

The proposed atomization system obviously requires delivery of thefluids 48, 49 to be used in the dispersion process at appropriateflow-rates. Thus:

(1) Both flow-rates should be adjusted for the system so that they liewithin the stable parameter window.

(2) The mass ratio between the flows should be compatible with thespecifications of each application. Obviously, the gas flow-rate can beincreased by using an external means in special applications (e.g.burning, drug inhalation) since this need not interfere with theatomizer operation.

(3) If the flow-rates are altered, the characteristic time for thevariation should be shorter than the hydrodynamic residence times forthe liquid and gas in the microjet, and smaller than the reciprocal ofthe first natural oscillation frequency of the drop formed at the end ofthe feeding piece.

(4) Therefore, the gas and liquid can be dispensed by any type ofcontinuous delivery system (e.g. a compressor or a pressurized tank theformer and a volumetric pump or a pressurized bottle the latter).

(5) The atomizer can be made from a variety of materials (metal,polymers, ceramics, glass).

Spectrographic Analysis

An embodiment of the type shown in FIGS. 1A and 1B can be modified toprovide an analytical device. A signal emitter (e.g. infrared) ispositioned such that the signal is directed at and through the stablecapillary microjet of fluid coming from the feeding source 1. A signalreceiving component is positioned opposite the emitter. Thus, the flowstream out of the feeding needle 1 is positioned directly between theemitter and receiver. Two feeding needles may be used so that one canprovide a flow stream of, for example, the solvent in which the materialto be analyzed is dissolved. Two readings are made simultaneously andthe reading of the solvent is subtracted away by microprocessor devicesof the type known to those skilled in the art to obtain a true analysisof only the material of interest.

In addition to analysis of any compound dissolved or suspended in asolvent the methodology can be used to analyze materials such as bodyfluids e.g. blood or urine. The methodology can be adapted to work in awide range of different systems, e.g. see U.S. Pat. No. 5,126,022 issuedJun. 30, 1992 and patents and publications cited therein. The presentinvention does not need to use electrical fields to move chargedmolecules as is required by many other systems. Thus, non-polarmolecules can be moved, via the present invention, through the capillarymicrojet. Because of the manner in which the stable capillary microjetis formed and maintained materials such as large proteins, nucleotidesequences, cells, and other biomaterials are not destroyed by physicalstresses.

Drug Delivery Device

A device of the invention may be used to provide particles for drugdelivery, e.g. the pulmonary delivery of aerosolized pharmaceuticalcompositions. The device would produce aerosolized particles ofpharmaceutically active drug for delivery to a patient by inhalation.The device is comprised of a liquid feeding source such as a channel towhich formulation is added at one end and expelled through an exitopening. The feeding channel is surrounded by a pressurized chamber intowhich gas is fed and out of which gas is expelled from an opening. Theopening from which the gas is expelled is positioned directly in frontof the flow path of liquid expelled from the feeding channel. Variousparameters are adjusted so that pressurized gas surrounds liquid flowingout of the feeding channel in a manner so as to maintain a stablecapillary microjet of liquid until the liquid exits the pressure chamberopening and is aerosolized. The aerosolized particles having a uniformdiameter in the range of about 1 to 5 microns are inhaled into apatient's lungs and thereafter reach the patient's circulatory system.

Production of Dry Particles

The method of the invention is also applicable in the mass production ofdry particles. Such particles are usefull in providing a highlydispersible dry pharmaceutical particles containing a drug suitable forpulmonary delivery. The particles formed of pharmaceutical areparticularly useful in a dry powder inhaler due to the small size of theparticles (e.g. 1, 2, 3, 4, or 5 microns in diameter) and conformity ofsize (e.g. 3 to 30% difference in diameter) from particle to particle.Such particles should improve dosage by providing accurate and preciseamounts of dispersible particles to a patient in need of treatment. Dryparticles are also useful because they may serve as a particle sizestandard in numerous applications.

For the formation of dry particles, the first fluid is preferably aliquid, and the second fluid is preferably a gas, although two liquidsmay also be used provided they are generally immiscible. Atomizedparticles within a desired size range (e.g., 1 micron to about 5microns) The first fluid liquid is preferably a solution containing ahigh concentration of solute. Alternatively, the first fluid liquid is asuspension containing a high concentration of suspended matter. Ineither case, the liquid quickly evaporates upon atomization (due to thesmall size of the particles formed) to leave very small dry particles.

Fuel Injection Apparatus

The device of the invention is useful to introduce fuel into internalcombustion engines by functioning as a fuel injection nozzle, whichintroduces a fine spray of aerosolized fuel into the combustion chamberof the engine. The fuel injection nozzle has a unique fuel deliverysystem with a pressure chamber and a fuel source. Atomized fuelparticles within a desired size range (e.g., 5 micron to about 500microns, and preferably between 10 and 100 microns) are produced from aliquid fuel formulation provided via a fuel supply opening. The fuel maybe provided in any desired manner, e.g., forced through a channel of afeeding needle and expelled out of an exit opening of the needle.Simultaneously, a second fluid contained in a pressure chamber whichsurrounds at least the area where the formulation is provided, e.g.,surrounds the exit opening of the needle, is forced out of an openingpositioned in front of the flow path of the provided fuel, e.g. in frontof the fuel expelled from the feeding needle. Various parameters areadjusted to obtain a stable fuel-fluid interface and a stable capillarymicrojet of the fuel, which allows formation of atomized fuel particleson exiting the opening of the pressurized chamber.

Fuel injectors of the invention have three significant advantages overprior injectors. First, fuel never contacts the periphery of the exitorifice from which it is emitted because the fuel stream is surroundedby a gas (e.g. air) which flows into the exit orifice. Thus, clogging ofthe orifice is eliminated or substantially reduced. Second, the fuelexits the orifice and forms very small particles which are substantiallyuniform in size, thereby allowing faster and more controlled combustionof the fuel. Third, by using the methods described herein, the amount ofenergy needed to produce aerosolized particles of fuel -is substantiallyless than that required by other methods.

Microfabrication

Molecular assembly presents a ‘bottom-up’ approach to the fabrication ofobjects specified with incredible precision. Molecular assembly includesconstruction of objects using tiny assembly components, which can bearranged using techniques such as microscopy, e.g. scanning electronmicrospray. Molecular self-assembly is a related strategy in chemicalsynthesis, with the potential of generating nonbiological structureswith dimensions as small as 1 to 100 nanometers, and having molecularweights of 10⁴ to 10¹⁰ daltons. Microelectro-deposition and microetchingcan also be used in microfabrication of objects having distinct,patterned surfaces.

Atomized particles within a desired size range (e,g., 0.001 micron toabout 0.5 microns) can be produced to serve as assembly components toserve as building blocks for the microfabrication of objects, or mayserve as templates for the self-assembly of monolayers for microassemblyof objects. In addition, the method of the invention can employ anatomizate to etch configurations and/or patterns onto the surface of anobject by removing a selected portion of the surface.

Aeration of Water

More fish die from a lack of oxygen than any other cause. Fish exposedto low oxygen conditions become much more vulnerable to disease,parasites and infection, since low oxygen levels will (1) lower theoxidation/reduction potential (ORP) (2) favor growth of disease causingpathogens and (3) disrupt the function of many commercially availablebiofilters. Moreover, stress will reduce the fish activity level, growthrate, and may interfere with proper development. A continuous healthyminimum of oxygen is approximately a 6 parts per million (ppm)oxygen:water ratio, which is approximately 24 grams of dissolved oxygenper 1000 gallons of water. Fish consume on average 18 grams of oxygenper hour for every ten pounds of fish. Low level stress and poor feedingresponse can be seen at oxygen levels of 4-5 ppm. Acute stress, nofeeding and inactivity can be seen at oxygen levels of 2-4 ppm, andoxygen levels of approximately 1-2 ppm generally result in death. Thesenumbers are merely a guideline since a number of variable (e.g., watertemperature, water quality, condition of fish, level of other gasses,etc.) all may impact on actual oxygen needs.

Proper aeration depends primarily on two factors: the gentleness anddirection of water flow and the size and amount of the air bubbles. Withrespect to the latter, smaller air bubbles are preferable because they(1) increase the surface are between the air and the water, providing alarger area for oxygen diffusion and (2) smaller bubbles stay suspendedin water longer, providing a greater time period over which the oxygenmay diffuse into the water.

The technology of the invention provides a method for aerating water forthe proper growth and maintenance of fish. A device of the invention forsuch a use would provide an oxygenated gas, preferably air, as the firstfluid, and a liquid, preferably water, as the second fluid. The airprovided in a feeding source will be focused by the flow of thesurrounding water, creating a stable cusp at the interface of the twofluids. The particles containing the gas nucleus, and preferably airnucleus, are expelled into the liquid medium where aeration is desired.When the first fluid of the invention is a liquid, and the second fluidis a gas, the inertia of the first fluid is low, and the gas abruptlydecelerates very soon after it issues from the cusp of the attacheddroplet In such an instance, the microjet is so short that it is almostindistinguishable from the stable cusp.

EXAMPLES

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how tomake and use the present invention, and are not intended to limit thescope of what the inventors regard as their invention nor are theyintended to represent that the experiments below are all or the onlyexperiments performed. Efforts have been made to ensure accuracy withrespect to numbers used (e.g. amounts, temperature, etc.) but someexperimental errors and deviations should be accounted for. Unlessindicated otherwise, parts are parts by weight, molecular weight isweight average molecular weight, temperature is in degrees Centigrade,and pressure is at or near atmospheric.

The properties of sixteen different liquids are provided in Table 1

TABLE 1 Liquids used and some of their physical properties at 24.5° C.(ρ: kg/m³, μ: cpoise, γ: N/m). Also given, the symbols used in theplots. Liquid ρ μ γ Symbol Heptane 684 0.38 0.021 ∘ Tap Water 1000 1.000.056 ⋄ Water + glycerol 90/10 v/v 1026 1.39 0.069 Δ Water + glycerol80/20 v/v 1052 1.98 0.068 ∇ Isopropyl alcohol 755.5 2.18 0.021 x Water +glycerol 70/30 v/v 1078 2.76 0.067  Water + glycerol 60/40 v/v 11044.37 0.067  Water + glycerol 50/50 v/v 1030 6.17 0.066 ∘ 1-Octanol 8277.47 0.024 ⋄ Water + glycerol 40/60 v/v 1156 12.3 0.065 Δ Water +glycerol 35/65 v/v 1167 15.9 0.064 ∇ Water + glycerol 30/70 v/v 118224.3 0.064 x Water + glycerol 25/75 v/v 1195 38.7 0.063 + Propyleneglycol 1026 41.8 0.036 

The liquids of Table 1 were forced through a feeding needle of the typeshown in FIGS. 1A and 1B. The end 5 of the feeding needle had aninternal radius R₀. The exit orifice 6 had a diameter D and the wall ofthe pressure chamber 3 had a thickness of L. Three different deviceswere tested having the following dimensions: (D=0.15, 0.2, and 0.3 mm;L=0.1, 0.2 and 0.35 mm; R₀+0.2, 0.4, and 0.6 mm, respectively), andseveral distances H from the tube mouth to the orifice ranging fromH=0.5 mm to H=1.5 mm have been used. The jet diameter was measured atthe hole exit and was plotted as a function of the pressure differenceΔP_(g) and flow rate Q respectively. Although this technique allows forjet diameters even below one micron, larger flow rates and diametershave been used in this study to diminish the measuring errors.

In order to collapse all of the data, we define a reference flow rateQ_(n) and diameter d_(n) based on the minimal values, from expressions(3) and (5), that can be attained in stable regime for a given ΔP_(g):$\begin{matrix}{{Q_{o} = \left( \frac{\gamma^{4}}{\rho_{1}\Delta \quad P_{g}^{3}} \right)^{1/2}},\quad {d_{o} = \frac{\gamma}{\Delta \quad P_{g}}}} & (6)\end{matrix}$

These definitions provide the advantage of a nondimensional expressionfor (5), as

d _(j) /d _(a)=(8/π²)^(¼() Q/Q _(o))^(½,)  (7)

which allows for a check for the validity of neglecting the surface termin (4) (i.e., Q/Q_(o) should be large).

Notice that if the measured d_(j) follows expression (5), the surfacetension cancels out in (7). Also notice that d_(j)/d_(o)∝We/2.

350 measured values of d_(j)/d_(o) versus Q/Q₀ are plotted in FIG. 5. Acontinuous line represents the theoretical prediction (7), independentof liquid viscosity and surface tension. The use of different hole andtube diameters as well as tube-hole distances does not have anyappreciable influence on d_(j). The collapse of the experimental dataand the agreement with the simple theoretical model is excellentFinally, the experimental values of Q are at least four times large thanQ_(o) (being in most cases several hundreds times larger), whichjustifies the neglect of the surface tension term in Eq. (4).

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation, material, composition of matter, process, processstep or steps, to the objective, spirit and scope of the presentinvention. All such modifications are intended to be within the scope ofthe claims appended hereto.

What is claimed is:
 1. A method of manufacturing coated particles, comprising the steps of: (a) forcing a liquid formulation comprising a pharmaceutically active drug through a channel of a first feeding source in a manner which causes a stream of the liquid drug to be expelled from a first exit opening at a first velocity; (b) forcing a liquid comprising a coating material through a second channel concentrically positioned around the first channel in a manner which causes a stream of the liquid coating material to be expelled from a second exit opening at a velocity which is substantially the same as the first velocity whereby the stream of coating material is concentrically positioned around the stream of liquid drug; (c) forcing a gas through a pressure chamber in a manner which causes the gas to exit the pressure chamber from an exit orifice positioned downstream of the concentrically positioned streams of liquid drug and coating material; wherein the density of the liquid formulation comprising the pharmaceutically active drug is substantially the same as the density of the liquid comprising the coating material, and the gas focuses the concentrically positioned streams to a stable unified jet which flows out of the chamber exit orifice and breaks up into a first group of coated particles of the pharmaceutically active drug coated with the coating material; and (d) repeating the steps (a)-(c) in a manner such that the coating on a second group of coated particles has a different thickness from the coating of the first group of coated particles.
 2. The method of claim 1, wherein the repeating (d) of steps (a)-(c) is carried out in a separate manufacturing event.
 3. The method of claim 1, wherein the repeating step (d) is carried out a plurality of times to obtain a plurality of groups of coated particles.
 4. The method of claim 3, wherein each of the groups of coated particles has different coating thickness and combined groups of coated particles provide controlled release of the pharmaceutically active drug.
 5. The method of claim 1, wherein the stable unified jet formed in each step (c) is comprises a diameter d_(j) at a given point A in the stream characterized by the formula: $d_{j} \cong {\left( \frac{8\rho_{1}}{\pi^{2}\Delta \quad P_{g}} \right)^{1/4}Q^{1/2}}$

wherein d_(j) is the diameter of the stable unified jet, ≅ indicates approximately equally to where an acceptable margin of error is ±10%, ρ₁ is the average density of the liquid of the unified jet and ΔP_(g) is change in gas pressure of gas surrounding the stream at the point A and Q is the total flow rate of the stable unified jet.
 6. The method of claim 5, wherein d_(j) is a diameter in a range of about 1 micron to about 1 mm.
 7. The method of claim 5, wherein the stable unified jet formed in each step (c) has a length in a range of from about 1 micron to about 50 mm.
 8. The method of claim 5, wherein the stable unified jet formed in each step (c) is maintained, at least in part, by tangential viscous stresses exerted by the gas on a surface of the jet in an axial direction of the jet.
 9. The method of claim 5, wherein the stable unified jet formed in each step (c) is further characterized by a slightly parabolic axial velocity profile.
 10. The method of claim 5, wherein each group of particles of the pharmaceutically active drug coated with the coating material are characterized by having the same diameter with a deviation in diameter from one particle to another in a range of from about ±3% to about ±30%.
 11. The method of claim 10, wherein the deviation in diameter from one particle to another in each group of particles is in a range of from about ±3% to ±10%.
 12. The method of claim 1, wherein a given coated particle in each group of particles has a diameter in a range of about 0.1 micron to about 100 microns and other particles in that group have the same diameter as the given particle with a deviation of about ±3% to about ±30%.
 13. The method of claim 1, wherein ΔP=P₀−P₁, the difference in pressure through the pressure chamber exit orifice, is equal to or less than twenty times the surface tension of the liquid comprising the coating material with the gas, divided by the radius of the stable unified jet. 